Abstract: The research at home and abroad on sludge reduction by ozonation is reported in detail. Most attention is paid to the principle of sludge reduction by ozonation，efficiency of sludge reduction，relationship between efficiency and ozone dosage，as well as the effect of return sludge ozonation on wastewater treatment running and sludge settling and dewatering characteristics. Sludge reduction by ozonation can minimize excess biomass，have less impact on the effluent water quality，further more，can improve the system’s denitrification. Sludge reduction by ozonation shows a wide application foreground.